[dev.typeparams] cmd/compile/internal/types2: move embedding positions from Checker to Interface

This change moves the position information to the place where it
is actually used. It also simplifies getting rid of it after use.

In the process, fixed a latent bug: Before this CL, embedded types
were sorted, but the corresponding embedding positions were not.
Removed the sorting altogether as it is not needed for type-checking.

Change-Id: I48003f317196d814326424430336b6cb222fdee6
Reviewed-on: https://go-review.googlesource.com/c/go/+/331514


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>